Generally, open gullies are known, from gullies on roofs or in roads as well as in wash and laundry drains and sinks. The basic principle is that liquid flows due to its own weight by “gravity flow,” and these gullies can be called self-draining. These gullies are open and allow air to enter the outlet or drainage system, thereby limiting the amount of liquid to be drained. In addition, the employment of a manifold may hinder liquid flowing from a gully at a higher level through a lower gully.
FIG. 1 of the drawings shows a system of the gravity flow gully type which is, due to the above mentioned reason, preferably avoids use of a manifold. These types of gullies have their own outlets which are gathered in bottom pipes and are fed to a basin or direct to the drainage or sewer pipeline network.
A prior art referred to as NO Application No. 17591 of the same inventor discusses a gully having elongated channel parts extending radially from a central part.
There are also vacuum gullies, also called total flow gullies, where gases like air are excluded from the flow. The technical effect of this is that a liquid column is established from the gully to the outlet, the complete weight of said column generating a heavy suction to handle lager amounts of water than open gullies. In addition, this enables the use of manifolds so as to save pipes and simplify portions of the structure. Such systems are often called “full-bore flow” or “syphonic” (or “siphonic”). There are, however, a number of problems connected to such gullies.                The gully head is more complex, as the head comprises a housing part having a roof and forms an air lock, the roof defining the maximum height for the opening into the housing part.        The gully may include a throttle or choke disk, often in the form of a ring or a plate having a hole, arranged in the gully bottom and limiting/throttling the outlet dimension.        Several throttle disks of various dimensions have to be produced.        The gully head is easily blocked by extraneous matter like leaves, and dirt and smaller particles that gather together to form a kind of clay.        The system is complicated to calculate and dimension.        The system is, when installed, sensitive to even minor changes, like new superstructures or the adjustments of outlet pipes or constructional alterations, leading to changes in the amount of water between the gullies. Then, new calculations and adjustments for new throttle disks or gully dimensions have to be made, and then such disks and gullies must be constructed.        Gullies have mainly been arranged in the same heights, and if gullies in different floors of a building are to be connected together, the dimensioning has to be calculated and the connection has to be carried out further down to the outlet.        The gullies can be adapted to different amounts of water by use of throttle disks, but only to a limit.        If the amounts of water are too small, the outlet pipes will no longer be self-cleaning, so that the outlets over the time may become fouled/clogged unless regularly flushed.        
FIG. 2 shows a system with a vacuum gully. It should be mentioned that the outlet system for such gullies comprises pipes arranged horizontally, that is, without any inclination. Because the outlet pipes in this case can be arranged horizontally (without inclination), these pipes are accommodated just under the ceiling, and are assembled to turn down at only one point in their connections. Due to this, the pipe arrangement in the ground will be at a minimum, which is particularly favorable when the building is on a rock fundament.
In a first period, a vacuum gully will operate like a gravity flow gully, and only when the water level rises over the roof height of the inlet, gas will be expelled so the gully starts operating as a vacuum gully. The draining ability increases dramatically and is maintained until the water level has been reduced so far that also air (a gas) is drawn in, whereby the gully again changes to operate as a gravity flow gully. A vacuum gully having a 75 mm diameter can, as an example, handle 10 liters per second at a water level of 35 mm, and 19 liters per second at a water level of 55 mm.
When several vacuum gullies are connected together, the air intake from one gully will be sufficient for that liquid column to no longer be unbroken, so that all the gullies start to operate as gravity flow gullies. This can be remedied by using throttle disks so that the gullies, during operation, will be drained approximately at the same time, so that the complete system may operate as a vacuum gully for as long as possible.
Vacuum gullies can have an opening of 11 mm between surface and roof, but without a throttle disk. Also, vacuum gullies can be used with an opening of 19 mm and have throttle disks. A lower opening means that the gully operates as a vacuum gully even with a lower water level, but has the drawback that the resistance against flowing through also becomes higher.
Common for gullies of the gravity flow and vacuum gullies is that warm air rising from the outlet causes ice build-up on the roof surface. This happens because the gully remains at temperatures above the freezing point while the snow on the roof is holding temperatures below freezing. Ice will then build up in the area around zero degrees (0° C.). The build-up of ice will normally become greater for gravity-flow gullies due to the employment of greater outlet pipes that normally dissipate more heat and therefore melt more slow, which leads to the build-up of more ice.
The heat is generated by at least portions of the outlet pipes are in frost-free ground, and is brought up through building constructions having higher temperature. Warm air therefore ascends from the outlets and heats up the gullies in varying degrees, but common for all the gullies is that they receive a surplus heat, keeping them free from frost so that they are not frozen completely. This is a great benefit for the gullies themselves, as the entire outlet would become blocked if the gullies are frozen. The drawback is that the surplus heat also melts the snow around the gully so that the melted water can build up ice blocks around the gully on the roof surface.